Regarding semiconductor memory devices (memories), studies on MRAM (Magnetic Random Access Memory), ReRAM (Resistive Random Access Memory) and PCRAM (Phase Change Random Access Memory), and the like are performed. In these memories, a resistance variable element is structured such that two states, a high resistance state and a low resistance state, of electric resistance occur, with which binary values (“0” and “1”) can be stored. The MRAM stores information using a change in magnetic resistance accompanying a change in magnitude of current. The ReRAM stores information using a change in resistance accompanying a change in magnitude of current and voltage. The PCRAM stores information using a change in resistance accompanying a change in crystal phase.
In the MRAM, for example, a magneto-resistance variable element such as TMR (Tunnel Magneto-Resistance) element is provided in each memory cell. The TMR element is provided with a ferromagnetic layer (fixed layer) in which the direction of magnetization is fixed, a ferromagnetic layer (free layer) in which the direction of magnetization is variable, and an insulating layer (barrier layer) formed between these layers. Such a TMR element turns to a low resistance state when the direction of magnetization of the free layer coincides (in parallel) with the direction of magnetization of the fixed layer, and turns to a high resistance state when the direction of magnetization of the free layer is reverse (antiparallel) to the direction of magnetization of the fixed layer. As a method of changing the direction of magnetization of the free layer, there is a method to pass a current through a specific wiring (occasionally called a write word line) provided in each memory cell, so as to apply magnetization accompanying this current to the free layer. A structure employing this method is occasionally called a write wiring type. Further, there is also a method to pass a current directly to the TMR element so as to use a spin-torque effect generated accompanying this current. A structure employing this method is occasionally called a spin-injection type. In addition, the current needed for changing the direction of magnetization of the free layer is proportional to the size of the element. Unlike the structure employing the method to apply magnetization accompanying a current, the spin-injection type does not need the wiring (write word line) for controlling the direction of magnetization. Accordingly, this type is preferable for densification. Further, as described above, since the current needed for changing the direction of magnetization of the free layer is proportional to the size of the element, it is possible to reduce the current needed for writing information as the device is miniaturized. Therefore, in recent years, MRAMs of the spin-injection type are gaining attention.
In the ReRAM, a resistance variable element in which an electric resistance varies accompanying a change in magnitude of current and voltage is provided in each memory cell. The resistance variable element is provided with two electrodes and a resistance variable film provided between the electrodes. The resistance of the resistance variable film varies corresponding to the magnitude of current, and an oxide containing transition metal such as a nickel oxide is used for its material.
The PCRAM is occasionally called a phase change memory. In the PCRAM, a resistance variable element in which electric resistance changes accompanying a change in crystal phase is provided in each memory cell. The resistance variable element is provided with a phase change layer formed of a material which turns to an amorphous state or a crystal state accompanying a temperature change or the like. Such a phase change layer turns to a high resistance state when it is in the amorphous state and to a low resistance state when it is in the crystal state. A method for changing the phase is to pass a current and use the Joule heat generated by this current. When the phase change layer is in the amorphous state (high resistance state), a voltage is applied to this layer to pass a current. When the current increases to a certain degree, the phase change layer is heated by the Joule heat and changes to the crystal state (low resistance state). When the voltage is decreased from this state, the crystal state is maintained. On the other hand, when the phase change layer is in the crystal state (low resistance state), passing a current at a predetermined level or higher to this layer causes the crystal to melt, and the phase change layer changes to the amorphous state (high resistance state). Moreover, the voltage increases accompanying the change to the high resistance state, and the current decreases. Then, as the voltage is decreased from this state, the amorphous state is maintained.
In this way, conventionally, binary information is stored in one memory cell.
On the other hand, to store quaternary information in one memory cell, a structure in which two phase change layers are provided in one memory cell is proposed.
However, although there are four types of combinations of states of phases in the two phase change layers, it is not possible to change among these four types of states only by single processing. For example, although it is possible to change from one state to another state by single processing, it is not possible to cause a reverse change by single processing, and it is occasionally necessary to further go through another state in the middle. Accordingly, control becomes complicated.    Patent Document 1: Japanese Patent No. 3531628    Patent Document 2: Japanese Laid-open Patent Publication No. 2004-158804    Patent Document 3: Japanese Laid-open Patent Publication No. 2004-363604    Patent Document 4: Japanese Laid-open Patent Publication No. 2005-260014